Light: Reflection, Mirrors and the Human Eye
Light makes objects visible through reflection from surfaces, dispersion into colours, and the working of the human eye.
UPSC Prelims periodically lifts basic-science MCQs straight from this NCERT chapter — laws of reflection, luminous vs illuminated objects, eye anatomy (rods/cones, blind spot), dispersion and persistence of vision. For Mains it underpins GS-III science-and-technology fundamentals, while the eye-disease content (cataract, vitamin-A deficiency) feeds the GS-II/GS-III public-health theme of preventable blindness.
Understand the chapter
How We See: Luminous and Illuminated Objects
We see an object only when light from it enters our eyes; the eye alone cannot see in the dark. This light is either emitted by the object or reflected off it. Bodies that produce their own light are luminous; those that merely reflect light from another source are illuminated.
- Luminous: Sun, fire, candle flame, electric lamp.
- Illuminated: Moon (reflects sunlight) and most everyday objects.
- No light entering the eye means no vision — that is why we cannot see in the dark.
Laws of Reflection
When light strikes a surface it bounces back as the reflected ray. The normal is the line drawn perpendicular to the surface at the point of incidence; the angle of incidence (i) lies between the incident ray and the normal, and the angle of reflection (r) between the reflected ray and the normal. Two laws govern reflection at every surface.
- Law 1: angle of incidence = angle of reflection (i = r).
- Law 2: incident ray, reflected ray and the normal lie in the same plane.
- A 'ray' is an idealisation of a narrow beam of light.
Image Formed by a Plane Mirror
Reflected rays from a point object only appear to meet behind the mirror, so a plane mirror forms a virtual image that cannot be caught on a screen. The image is erect, the same size as the object, and lies as far behind the mirror as the object is in front. The left and right of the object are swapped — lateral inversion.
- Properties: virtual, erect, same size, equal distance behind, laterally inverted.
- Lateral inversion is a left-right swap, not an up-down one.
- Cannot be obtained on a screen, unlike a real image.
Regular vs Diffused Reflection
Reflection from a smooth surface like a mirror keeps parallel rays parallel — regular reflection — and this is what forms clear images. A rough surface scatters parallel rays in different directions, giving diffused (irregular) reflection and no clear image. Crucially, the laws of reflection still hold at every point; only the surface irregularities cause the scatter.
- Regular: smooth surface, parallel rays stay parallel, forms images.
- Diffused: rough surface, rays scattered in many directions, no image.
- Diffused reflection does NOT violate the laws of reflection.
Reflecting Again: Multiple Images and Devices
A reflected ray can itself be reflected again, which lets us see around corners and behind us. Two plane mirrors set at an angle multiply the images: as the angle shrinks the count rises, and parallel mirrors give infinitely many. This principle powers periscopes and kaleidoscopes.
- Images from two mirrors at angle θ = (360/θ) - 1; at 90 degrees = 3 images.
- Parallel mirrors (0 degrees) = infinite images.
- Periscope: two plane mirrors; used in submarines, tanks and bunkers.
- Kaleidoscope: three mirror strips forming a prism; patterns never repeat.
Sunlight and Dispersion
Sunlight is white light made up of seven colours. When it passes through a prism (or the mirror-and-water arrangement) it splits into its component colours — dispersion. A rainbow is the natural display of the dispersion of sunlight.
- White light = seven colours (VIBGYOR).
- Dispersion = splitting of white light into its colours by a prism.
- Rainbow = natural dispersion of sunlight.
The Human Eye: Structure, Vision and Care
Light enters through the transparent cornea, passes the pupil (whose size the iris adjusts), and the convex lens focuses it onto the retina. The retina's rods sense dim light while cones sense bright light and colour; signals travel via the optic nerve to the brain. Defects, diseases and the persistence of vision complete the picture.
- Iris gives the eye its colour and controls the pupil and light entering.
- Blind spot: optic-nerve junction with no sensory cells, so no vision there.
- Persistence of vision is about 1/16 second; cinema runs at 24 frames per second.
- Cataract = cloudy lens (treated with an artificial lens); vitamin-A deficiency harms vision.
Key terms
- Luminous object
- A body that emits its own light, e.g. the Sun, fire, a candle, an electric lamp.
- Illuminated object
- A body seen only by the light it reflects from another source, e.g. the Moon.
- Normal
- The line drawn perpendicular (90 degrees) to a surface at the point where light strikes it.
- Regular reflection
- Reflection from a smooth surface where parallel incident rays stay parallel and form a clear image.
- Diffused (irregular) reflection
- Scattering of parallel rays in many directions by a rough surface; no clear image, yet the laws of reflection still hold.
- Lateral inversion
- The left-right reversal of an object as seen in its plane-mirror image.
- Virtual image
- An image formed where reflected rays only appear to meet; it cannot be captured on a screen.
- Dispersion
- The splitting of white light into its seven constituent colours, as by a prism.
- Persistence of vision
- The retina's retention of an image for about 1/16 second, which makes rapid still pictures appear to move.
- Blind spot
- The point where the optic nerve meets the retina; it has no sensory cells, so no image forms there.
Must-know facts exam-ready
- First law of reflection: angle of incidence = angle of reflection (i = r).
- Second law of reflection: the incident ray, the reflected ray and the normal all lie in the same plane.
- A plane-mirror image is virtual, erect, same-size, laterally inverted, formed as far behind as the object is in front, and cannot be taken on a screen.
- Two mirrors inclined at angle θ form (360/θ) - 1 images; at 90 degrees this gives 3 images, and parallel mirrors give infinite images.
- Luminous objects emit their own light (Sun, fire, candle, electric lamp); illuminated objects shine by reflected light (the Moon).
- White sunlight splits into seven colours (VIBGYOR) by dispersion; a rainbow is natural dispersion.
- The iris gives the eye its colour and controls the pupil; the eye lens is convex (thicker at the centre) and focuses the image on the retina.
- The retina has rods (sensitive to dim light) and cones (sensitive to bright light and responsible for colour).
- The blind spot is the optic-nerve–retina junction with no sensory cells, so no vision is possible there.
- The least distance of distinct vision for a normal eye is about 25 cm.
- Persistence of vision is about 1/16 second; cinema is shown at 24 pictures per second (above the 16 per second threshold).
- Cataract is a cloudy/opaque eye lens, cured by removing it and inserting an artificial lens; vitamin-A deficiency harms vision (night blindness).
Memory tricks remember it for good
Traps to avoid
- Diffused (irregular) reflection does NOT break the laws of reflection — the laws hold at every point; only the surface is rough.
- The Moon is an illuminated object, not luminous — it only reflects sunlight.
- A plane-mirror image is virtual and cannot be caught on a screen; do not call it real.
- Lateral inversion swaps left and right, NOT up and down.
- Rods work in dim light (no colour) and cones in bright light with colour — these are frequently reversed.
- The 25 cm figure is the least distance of distinct vision for a normal eye and varies with age — it is not a universal fixed value.
Exam focus
🧠 Prelims angles
- Laws of reflection and identifying the normal, angle of incidence and angle of reflection in ray diagrams.
- Number of images formed by two inclined or parallel mirrors ((360/θ) - 1; parallel gives infinite).
- Luminous vs illuminated objects, with the Moon as the classic illuminated example.
- Eye anatomy and function — rods vs cones, blind spot, iris/pupil/retina, and least distance of distinct vision (25 cm).
- Dispersion, prism, VIBGYOR and rainbow; persistence of vision (1/16 s) and cinema at 24 frames per second.
- Optical devices — periscope (submarines, tanks, bunkers) and kaleidoscope; plus cataract and vitamin-A deficiency.
✍️ Mains angles GS-III
- Preventable blindness in India — cataract and vitamin-A deficiency as a public-health challenge.Use the chapter's eye-disease facts to discuss the National Programme for Control of Blindness and Visual Impairment (NPCBVI), screening, surgery and nutrition interventions.
- Applications of reflection and optics in technology and defence.Cite periscopes (submarines, tanks, bunkers), kaleidoscopes and mirrors to link basic optics to real-world science and technology.
- Screen time, lighting and eye health in the digital age.Apply eye-care principles — adequate light, normal reading distance, avoiding strain — to modern lifestyle and public-health messaging.
Last-minute revision tick as you recall
- i = r; incident ray, reflected ray and normal lie in one plane.
- Plane-mirror image: virtual, erect, same size, equal distance behind, laterally inverted.
- Diffused reflection: laws still hold; only the surface is rough.
- Two mirrors at θ give (360/θ) - 1 images; parallel mirrors give infinite.
- Luminous = own light (Sun, candle); illuminated = reflected (Moon).
- White light splits into seven colours (VIBGYOR) by dispersion; rainbow is natural dispersion.
- Eye path: cornea → pupil → lens → retina → optic nerve → brain; rods for dim light, cones for colour.
- Blind spot has no sensory cells; least distance of distinct vision is about 25 cm.
- Persistence of vision = 1/16 s; cinema = 24 frames/s; cataract = cloudy lens.
Distilled from NCERT Class 8 · Science (Class 8) for UPSC. Always cross-check facts with the original NCERT.